The present invention relates to implantable pacemakers, and more particularly to the output switch configuration used in an implantable pacemaker. Even more particularly, the present invention relates to an output switch for a pacemaker that includes built-in protection means for preventing undesirable transient currents from flowing therethrough whenever the output line of the pacemaker is subjected to a more negative potential than a stored stimulation potential.
The function of a pacemaker is to provide a stimulation pulse to a desired location in or on the heart whenever the heart needs to contract. Modern pacemakers allow the amplitude of the stimulation pulse to be programmably controlled. The desired amplitude of the stimulation pulse is stored within a storage element inside of the pacemaker, typically a capacitor. At the appropriate time this storage element is switchably connected through an output switch to an output terminal of the pacemaker. The output terminal of the pacemaker, in turn, is connected to the desired location of the heart by means of a suitable conductive lead. This lead has at its distal end an electrode tip through which the stimulation pulse is presented to the heart tissue or fluids. A suitable electrical return path from the heart back to the pacemaker is also provided either through the conductive body tissue and fluids back to the pacemaker case (unipolar pacing) or through a ring electrode positioned on the pacemaker lead near the electrode tip (bipolar pacing).
The output switch that is used to switchably connect the storage element to the output terminal of the pacemaker comprises a semiconductor solid state switch. In recent years, such switches have employed metal oxide semiconductor field effect transistor (MOSFET) devices. (For purposes of this application, the term "MOSFET" will be used to broadly describe all types of semiconductor switches wherein an applied gate voltage selectively inhibits or enhances a conductive channel between source and drain terminals.) MOSFET type switches are generally preferred over other types of semiconductor switches, such as bipolar transistors, because of their low power consumption. That is, a MOSFET switch device consumes power only during the transition from an ON to an OFF state, or from an OFF to an ON state. In contrast, a bipolar switch typically consumes power continuously while in an ON state. Further, MOSFET type devices are more compatible with, and in fact can be employed as part of, the complementary metal oxide semiconductor (CMOS) circuits that comprise the digital logic circuits of a modern pacemaker. That is, the output switch is advantageously realized using the same type of semiconductor switches that are used within the digital and analog portions of the pacemaker, thereby allowing the switch to be fabricated as part of the same semiconductor chip (on the same semiconductor substrate) as the other pacemaker circuits. This sharing of the semiconductor substrate allows the pacemaker circuits to be realized in a smaller space, thereby allowing the overall pacemaker to be smaller in size.
Unfortunately, the fabrication of MOSFET devices on a semiconductor substrate often results in the existence of parasitic bipolar transistors within the same substrate. (As used herein, the term "parasitic" refers to an unwanted circuit element that is an unavoidable adjunct of a wanted circuit element. That is, the bipolar transistor is an unwanted circuit element that is an unavoidable adjunct of a wanted circuit element--the MOSFET device.) The presence of a parasitic bipolar transistor can cause many problems. For example, a typical MOSFET configuration provides the equivalent of a bipolar PNP and NPN parasitic transistors, configured in an arrangement that is equivalent to a silicon controlled rectifier (SCR). This equivalent SCR, is triggered, can cause the substrate to become "latched" and inoperable. Accordingly, precautions must be taken in order to prevent any parasitic SCR configurations from triggering and latching.
The output switch circuits of modern pacemakers comprise suitable N-channel and P-channel MOSFET devices. With the use of such devices there also exists a parasitic NPN transistor having its collector coupled to the supply voltage V.sub.DD of the pacemaker, its emitter connected to the output line of the pacemaker, and its base connected to the stored stimulation voltage, SSV, of the pacemaker. Depending upon whether the storage element holding the stored stimulation charge is fully charged or discharged, the potential actually applied to the base of the parasitic transistor may vary between V.sub.DD and SSV. In any event, if the output line of the pacemaker is subjected to a potential that is lower than the potential applied to the base of the parasitic transistor, the bipolar NPN transistor will turn ON. When the bipolar transistor turns ON in this fashion, it connects the supply voltage, V.sub.DD, to the output line through its collector and emitter terminals. This action further connects the stored stimulation voltage, SSV, to the output line through the base-emitter terminals of the bipolar transistor. Thus, any potential on the output line that is lower than SSV turns this NPN parasitic bipolar transistor ON, thereby causing large currents to flow from V.sub.DD and/or SSV to the output terminal. Such large output currents, even if only transient in nature, can potentially damage the output switching circuitry. Moreover, the flow of such currents needlessly depletes the limited energy of the pacemaker's battery.
Attempts have been made in the prior art to limit the magnitude of any such large transient currents that might flow when a negative potential appears on the output line. However, such current limiting devices typically require the use of additional discrete components, which additional discrete components not only add to the bulk and expense of the pacemaker, but their effectiveness at limiting the magnitude of the transient currents may be dependent upon the magnitude of the triggering negative potential appearing on the output line of the pacemaker. Hence, whereas such limiting devices may be effective at limiting the value of the transient currents to less than destructive values when a nominal negative potential appears on the output line, they may not be effective when a greater than normal negative potential appears on the output line. Further, even if such limiting devices prevent the transient currents from reaching destructive amplitudes, they do not prevent the needless depletion of the pacemaker's battery.
Unfortunately, it is not uncommon for the output line of the pacemaker to have applied thereto a potential that is lower than the the stored stimulation voltage, SSV. The most common situation where this might occur would be the application of a defibrilation pulse or pulses to the patient wearing the pacemaker. Numerous other external factors could also cause the output line to go sufficiently negative to turn the parasitic transistor ON. Further, in addition to these external sources of a negative turn-on potential, it is also possible for internal sources of a negative potential to turn the parasitic transistor ON. Hence, both external and internal sources of a negative potential can possibly trigger potentially destructive transient currents within the output switching circuitry. Accordingly, there is a need in the pacemaker art for preventing such transient currents from flowing in the output switch circuitry in the event that the output line of the pacemaker is subjected to a negative potential, either from internal or external sources.